1. Field of the Invention
The present invention relates to a building board which is suited for use for constituting the external wall of building, and in particular to building board whose rabbeted portions are improved in waterproof property. The present invention also relates to a manufacturing apparatus for such a building board.
2. Description of the Related Arts
FIG. 14 shows a cross-sectional view in the vertical direction of a building board to be employed in the conventional horizontal siding work. On the surface of this building board 1, a pattern (design portion) 4 is provided and a top coating layer 5 formed on the pattern 4. At the upper horizontal edge portion a horizontally prolonged building board 1 is provided with an underlying rabbeted portion 3 for shiplap joint in the vertical direction, while at the lower horizontal edge portion of the building board 1 an overlying rabbeted portion 2 is formed. A caulking material 6 is applied to the underlying rabbeted portion 3. The caulking material 6 is pressed when the overlying rabbeted portion 2, which is formed on the lower horizontal edge portion of the building board 1 to be installed on the top side, is overlaid from the upside. Whereby it prevents a further invasion of rainwater from penetrating through gaps between the surface of the building board 1 located on the top and bottom. These overlying and underlying rabbeted portions 2 and 3 are formed by cutting work on the occasion of manufacturing the substrate.
FIG. 15 shows a structure in the vertical direction, which are constructed using a couple of building boards 1 shown in FIG. 14. In this case, these building boards 1 are joined together using metallic fixture 7. FIG. 16 shows a perspective view illustrating one example of the metallic fixture 7 shown in FIG. 15. This metallic fixture 7 comprises a substrate 8 and an engaging portion 9 which is extended forward from the substrate 8. A plurality of holes 11 are formed on the substrate 8 for attaching the substrate 8 to an underlying member by means of screws. The engaging portion 9 has a Y-shaped form in cross-section and branched into a downwardly inclined hook portion 9A and a upwardly inclined hook portion 9B which are to be engaged at the distal end portion with the overlying rabbeted portion 2 of the building boards 1. As shown in FIG. 15, the building boards 1 are fixed to the underlying member 10 (for example furring strip) by making use of the metallic fixture 7.
The overlying rabbeted portion 2 of the building board 1 shown in FIG. 14 is provided with an overlying tongue pattern portion at the front surface and an overlying tongue engaging portion at the back surface. The underlying rabbeted portion 3 is provided with an underlying tongue engaging portion formed such that it faces to the overlying tongue engaging portion and is covered by the overlying tongue pattern portion. This overlying rabbeted portion 2 cannot be coated even in a sealer coating to be performed by the ordinary roller coating on the back surface of the building board, the sealer coating being applied to improve the water-proofing of the back surface. Under the circumstances, there has been proposed a special sealer coating method as being disclosed in Japanese Patent Unexamined Publication 8-33861. Originally, the sealer coating however is not aimed to achieve a complete water-proofing at the back surface 12 of the building board 1, but is rather intended to accomplish a some degree of water-proofing at this portion of the building board 1. Therefore, it is essentially impossible in the aforementioned sealer coating to achieve a complete water-proofing of the back surface 12 of the building board 1.
On the other hand, the underlying rabbeted portion 3 is a portion whose surface is coated by spray coating, etc. from the front surface of the building board, so that a some degree of water-proofing is ensured. The reason why the expression of xe2x80x9ca some degreexe2x80x9d is adopted here is that, taking into consideration the fact that, in the case of building board consisting of a 3-ply structure comprising thin dense front and back surface layers, and a coarse thick central core layer (which may be a building board to be manufactured by means of a dry method), the coarse core layer comes to be exposed as a rabbeted portion as a result of the cutting work for forming the underlying rabbeted portion 3, thus this rabbeted portion becomes consequently poor in water-proofing as compared with the front and back surface layers.
With respect to the end portions (a slanted cut portion or a butt end of lumber) of both underlying and overlying rabbeted portions 2 and 3, the coating thereof becomes insufficient in spite of spray coating (the flow-down of coating material may be considered to be one of the causes). Therefore, there has been devised a special end-coating method as being disclosed in Japanese Patent Unexamined Publications 8-229501 or 10-15454. However, even in that method, the coated film at the end portions is generally insufficient in thickness, and hence, resulting in poor water-proofing as a matter of fact.
Meanwhile, there have been already proposed by the applicant several methods to greatly improve the water-proofing of building board by the application of a PE (polyethylene) backing sheet to the back surface of building board. However, any countermeasure to improve the water-proofing of the rabbeted portions has not been described, thus leaving a problem to be solved.
Therefore, an object of the present invention is to provide a building board which is capable of providing the rabbeted portions of building board with an absolute water-proofing.
Another object of the present invention is to provide an apparatus for manufacturing such a building board.
Namely, according to the present invention, there is provided a building board whose rabbeted portions of a cement substrate blank are covered by a plastic sheet.
The plastic sheet is preferably constituted by a polyethylene sheet since it can be manufactured at low cost, since it merely melts even if it is burnt without generating smoke, and since the combustion products thereof are only limited to water and carbon dioxide, giving no harm to lives.
The thickness of polyethylene sheet should preferably be in the range of 0.3 to 0.5 mm, which renders the polyethylene sheet to be well suited one in terms of strength, water-proofing and the adhesion thereof to the back surface of the cement substrate.
The building board manufacturing apparatus proposed by the present invention is provided with cover-forming heating means for covering the rabbeted portions of a cement substrate blank with a plastic sheet, which is effected through the heating of the plastic sheet.
The cover-forming heating means covers the rabbeted portions of building board with a plastic sheet after the trimming step of the backed layer, thereby enabling not only formation of a plastic sheet layer at the rabbeted portions with dimensional stability, but also further improvement in the water-proofing of the rabbeted portions.